Local switching systems process originating and terminating calls between customer lines connected to the switching system and trunks to other switching systems. Since most customer lines are connected only to a single local switching system, the availability of telecommunication service to a customer is dependent upon the continued operability of the local switching system.
Such systems are designed and constructed to provide continuing reliable service in the presence of faults. The reliability is sometimes achieved by providing identical duplicates of equipment in the more critical parts of the system. When a failing part is detected, the duplicate is placed in service and the failing part is removed. With care, the substitution of a duplicate for a failing part can occur without the loss of service to customers.
A distributed switching system is one comprised of a plurality of switch units which interface customer lines and trunks on a peripheral side of the unit and which interface a central switch arrangement on the other side of the unit. Calls between customers and/or trunks connected to different switch units are completed through the central switch. The AT&T system disclosed in the AT&T Technical Journal, July-August 1985, No. 6, Part Two, is a time division switching system where the switching function is distributed to a plurality of switching modules (SMs), each connected to a number of lines and/or trunks. Each SM provides connections among the lines and trunks connected to that module. Calls involving lines or trunks connected to different SMs are completed through a time-multiplexed switch (TMS) that interconnects the SMs. Each SM includes a control unit that controls the switching function of that SM. The system also includes a central control that controls the switching function of the TMS. All calls within the system require the selection of what is referred to as a network time slot. For inter-module calls, the network time slot is used for transmission from one SM, through the TMS, to another SM. The same network time slot is used for both directions of transmission. For intra-module calls, the network time slot is used within the SM to connect one line or trunk to another line or trunk. Two network time slots are used for intra-module calls, one for each transmission direction. Although the call processing function is distributed in the system in that the real-time intensive tasks associated with calls, e.g., signal processing, are performed by the switching module control units, the functions of selecting the network time slot and setting up the TMS path if the call is an inter-module call, are centralized, being performed by the central control. There are 512 channels (time slots) TS0 through TS511 between a given SM and the TMS. Setting up a path for an inter-module call between two SMs involves finding a channel that is available on the link to the first SM, for example TS44, and that has a corresponding available channel TS44 on the link to the second SM. The central control stores an availability bit map for each of the links to the SMs for use in performing the network time slot selection function. For each time slot marked not available on a given link, the central control also stores information defining the connection through the TMS to one of the other links. Network time slots are again marked available and the connection information deleted in the central control after a call ends. (For reasons of efficient processing, this operation may be deferred until a predetermined number of call disconnects, e.g., 15, occur or a predefined time elapses.) However, the path or connection through the TMS is not removed after the call ends. As described in U.S. Pat. No. 4,621,357 issued to S. Naiman et al. on Nov. 4, 1986, the TMS removes connections only as necessary to establish a new connection. The information defining established TMS connections is stored only within the TMS, and the network time slot selection function (also referred to herein as the available path selection function) is performed without reference to such information.
The three commonly assigned, copending applications of W. K. Cline et al. 07/356,823, M. T. Ardon 07/356,807, and M. T. Ardon 07/356,802 are directed to improved reliability arrangements for use in distributed switching systems such as the above-referenced AT&T system. The arrangements achieve substantially improved reliability by providing alternative means for completing inter-module calls between SMs when the TMS is unavailable. The alternative means provide inter-SM connectivity directly by way of trunks that interconnect the SMs, or indirectly via trunks connected from the SMs to a second switching office. Although such arrangements do sharply improve reliability, the costs of introducing, administering and maintaining additional trunks and modifying call processing software to route calls via those trunks are significant.
In view of the foregoing, a need exists in the distributed switching system art for an improved reliability arrangement which is more cost-effective than the trunk-based arrangements, but which still meets the enhanced reliability requirements of many switching system applications, preferably without any addition of hardware to the system.